Chemistry Notes

24.2.4 Oxidation Number Method

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The Oxidation Number Method

Concept

The Oxidation Number Method is a systematic approach to balancing redox equations. It relies on the principle of conservation of charge: The total increase in oxidation number must equal the total decrease in oxidation number.

Step-by-Step Guide

Example

Write a balanced ionic equation for the reaction of with

Step 1

Identify the Key Species

  • Identify the reactants (oxidising and reducing agents) and their products.
  • Only write down the ions involved and not the spectator ions.

Step 2

Balance Number of Atoms

  • Balance the number of atoms for the O.A. and R.A. and their products.

Step 3

ON and TON

  • Calculate the Oxidation Number (ON) for each species
  • Calculate the Total Oxidation Number (TON) for each species by multiplying the Oxidation Number with the stoichiometric constant (number in front of species).

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Step 4

Change in TON

  • Calculate the change in TON for the O.A. and R.A.
  • Simplify by dividing by a common factor if possible.

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Step 5

Cross-Multiply by Change in TON

  • Multiply the O.A and its product by the change in TON of the R.A
  • Multiply the R.A and its product by the change in TON of the O.A
  • Simplify the stoichiometric constants

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Step 6

Balancing Oxygen

  • Add to balance number of oxygen atoms

Step 7

Balancing Hydrogen

  • Add to balance number of hydrogen atoms

Step 8: If in Alkaline Medium

Neutralising

Only do this step if the reaction occurs in alkaline medium

  1. Carry out Step 7 as usual
  2. Add on both sides to cancel out , as such, number of added to each side should be equal to number of present.
  3. Combine the and to form water.
  4. Simplify by cancelling out excess molecules present on both sides of the equation.

Step 9: State Symbols

Adding State Symbols

Only add state symbols if asked to, if you add when not asked and make a mistake, your answer may pass for CON (contradictory) and lose marks

Disproportionation Reactions

Disproportionation

A disproportionation reaction is one in which the same species is oxidised and reduced.

Oxidation Number Method for Disproportionation

Lets say you have the species disproportionating to give and : Write two times in the reactants side: Use the usual Oxidation Number Method, and at the end just combine the ‘s together.

Graph View